UPDATED: Since some viewers asked for evidence to support this is a hoax, I am adding this update.
If you view the video in Chrome, you can watch it in slow motion by clicking the settings and selecting 0.25 playback speed. You can clearly see the edit transitions indicate the shark shots are not from the same footage.
Additionally, in the beginning of the video as the surfer circles the camera man, we get a near full 360-degree view of the sky, and it is cloudless. The reflection of the sun on the surfers head also indicates the sun is fairly high in the sky (not to mention the fact that the sun is never visible in the early footage). The footage of the bloody water has instances of the camera popping out of the water and showing a cloudy sky with a low sun. From similar angles earlier in the video the sun is never visible.
At about 15 seconds in when the shark appears a hang-bait being pulled to the left of the frame can also be viewed, which would indicate that the shark footage was shot during a baited dive. Also, at this point, you can see that the black lines at the top and bottom of the frame that are seen in the original kite-surfing footage are not present in the footage of the shark. There are some semi-transparent bands at the top and bottom of the frame, during the shark footage, which completely disappear when the video then transitions to the supposed victim.
There are actually some fairly impressive kite boarding videos on YouTube. Unfortunately, this is not one of them. According to the title and the video information, the video above purports to show a great white shark attack a kite surfer, when in reality it’s just a garbled mix of surface and underwater shots with a couple of snippets of white shark footage spliced in. Throw in some really bad acting and “blood” in the water, and you’ve got your supposed shark attack video. To the video creator’s credit, the transitions into and out of the white shark footage are fairly smooth.